Why is the Sky Yellow?

If you look across Puget Sound toward Seattle, WA, you will often notice a slight yellow haze above the city. This phenomenon is not unique to Seattle, but also to many other big cities (Los Angeles immediately comes to mind). Anywhere there are a lot of vehicles is a place where you are very likely to find this cloud over the land.

Why It Matters

The word “smog” comes from its two major components smoke + fog. In big cities this sometimes dense yellow material is created mainly by chemical processes involving cars, trucks, and other motor vehicles. In some cities, the problem is complicated by the fact that part of the smog may roll in from fair distant areas. Other factors include local geography. Cities surrounded by mountains do not get good air movement, so smog is trapped and cannot be dissipated well.

A direct link between vehicular traffic and smog can be seen. In many places, more intense formation of NOx materials correlates well with the number of vehicles on the road. Peak amounts of these compounds can be seen at rush hour in the morning when everyone is going to work, and a lower peak in the afternoon/evening as people go home (usually over a longer time period than the morning rush hour).

The chemistry of smog is rather complex. The process begins when nitrogen in the air reacts with atmospheric oxygen to form nitric oxide. This reaction can easily take place at the high temperatures found in a car engine as the air needed for combustion of the fuel will also form NO. Then a series of oxidation-reduction reactions occur that are initiated by sunlight, leading to the formation of other nitrogen-oxygen compounds (collectively known as NOx). Ozone and nitric acid can also be formed as secondary products of these processes.

Because of the reactivity of many products of smog, prolonged exposure to the air can be harmful to health. Joggers are encouraged to restrict their outdoor activities when the smog level is high. Children playing outside are quite susceptible since their lungs are still developing. Anyone who has a respiratory ailment needs to be careful of their smog exposure.

Credit: Edward Dalmulder

Source: http://www.flickr.com/photos/edwarddalmulder/7716780200/

License: CC BY-NC 3.0

To deal with high levels of pollution, people in many cities have resorted to wearing face masks
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